Instabile opamp in Tascam 122MKIII?

Laszlo

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Jan 16, 2019
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Near Budapest, Hungary
Gear owned
Tascam Ampex Mechlabor
Guys,

I sold my Tascam 122MKIII, and before the final calibration, I realized quite some EMI noise coming through the channels. I planned to place decoupling capacitors earlier anyaway, so I decided to do so now. I put 0,1uF/50V ceramic capacitors on each opamps power rails, meanwile checking the results as I progressed, and it was promising. When I tested the machine with playing some music, an awfull cracking noise came through one of the channels, the VU meter danced madly, then I lost one channel. After testing the circuits I realised that I lost one repro channel (dual JFET+EQ opamp, circled on the pic). I suspect that the opamp got instabile and blew up..

I double checked each and every soldering of the newly added capacitors, all seems ok, and I have no idea, what caused the fault. I soldered the capacitors on the wired side of the PCB. It would be good to know before I replace the opamp and the dual JFET which also seems to go faulty.

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Guys,
No response received so far, never mind..I have worked on this issue since then. First, I checked the dual FETs, than the opamp. The opamp got damaged. I bought an IC socket converter board to use DIL opamp, put an IC socket on it and placed a NE5532 back. And it worked! ...for five minutes..then again the awful crckleing noise, and the meter dancig madly. Now I did not wait until the opamp gets burned again (but if it is, no problem, I have an IC socket :) ), and switched the play off. Due to the nature of noise and how it started again, I suspected bad soldering joints. I started to resolder the suspected components, and just realised, how volnurable are these 30 year old iny tiny soldering pads! As usual, after I resoldered all relavant components, I found the issue. It was not soldering, but a disintegrating trimmer pot in the EQ feedback loop of the opamp. Actually the plastic body of it is falling apart in several pieces! But it was not visible, only after desoldering it.
After replacement the cracling noise disapperaed for ever. But I found that there are 11 more similar trimmers on the main panel, and all of them show the same phisical signs of disintegration.
I hope this helps you guys, because thes seems to be a component aging issue, just like with the case of the bad caps in the motor board, and also in the Dolby S panels.
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I think you are coming upon the found out level and grade of some of these cost cutting type parts. I have come to determine that Marcon caps are not your friend just as 10V caps in any equipment should be replaced with 16V high grade ones. When you are working on the front line you find these things but most people do not want you to complain to them about it- what is the point? I replace a lot of pots of the carbon phenolic build with Bourns Cermet from Mouser. In the 122 Mk II pitch control pot problem I have used a linear Plastic conductor type. Yes age and cheap parts can be a headaches we all have to deal with as they never envisioned these decks to be in use past 7-10 years but they are for the most part pretty solid. Some of the older decks like the C-3RX still can be repaired for the most part- once we get rid of that conductive glue and on some units I had real low play gain. Changed out as a shotgun trial 5 Marcon caps and the deck started to be normal again. Funny how these low grade Marcon caps keep showing up. The Z7000 play card had two shorted Marcon on it too. I get 50 years old Nichicon caps in some Teac decks that still work- I wonder when they decided to cut corners further?
 
Early on in this diagnosis, it would have been good to put a scope on the Op Amp outs where you made the change. The 10MHz op amps like the NE5532 are entirely different than the 1 or 2 MHz op amps they used so in Engineering the Poles and Zeros will have changed and this then will require that the power supply be bypassed with good .1ufd caps- I don't like ceramic, but this is where they might work. The op amp that burns itself out is a sign of oscillation in the circuit that does not belong there and so to stabilize the circuit some small caps might be needed on the feedback loop of the gain stage to limit the bandwidth to 200KHz . Having a 10MHz bandwidth op amp really does no good and I wonder why people go putting a good part in the center of a bunch or regular ones and they say it sounds better. It is best to leave the Engineering up to the professionals rather than follow all these You Tube know nothings and 90 some percent of them are wrong.
 

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